Dysplastic nevi are thought to be direct precursors to melanoma as well as markers of persons at increased risk for melanoma. As such they define target lesions which are ideal for chemoprevention trials aimed at decreasing melanoma incidence and an accessible tissue in which the biochemical events of transformation to neoplasia can be studied. However, the design of a clinical chemoprevention of melanoma trial is hampered by lack of an animal model to test methods of intervention. Study of melanocytic dysplasia is limited by the paucity of methods to study melanocyte growth in an in vitro or in vivo setting. This proposal develops and characterizes a system for the study of human nevi by transplanting these lesions to congenitally athymic (nude) mice. Preliminary results demonstrate that dysplastic nevi survive transplantation to nub mice and that their dysplastic characteristics are maintained for 4 weeks after engraftment. This research will further characterized this system, set the stage for future studies of the cell and molecular biology of the melanocytes in these lesions, and initiate experiments to alter the dysplastic characteristics of grafted nevi thereby leading directly to clinical melanoma chemoprevention trials.